The present invention generally relates to the field of coating technologies, and more particularly, to an electrospark deposition apparatus and a method of controlling same.
Electrospark deposition (ESD) is a pulsed-arc, micro-welding process that uses short-duration, high-current electrical pulses to deposit a consumable electrode material on a conductive workpiece. ESD processes typically involve very high spark frequencies with spark durations lasting only a few microseconds, and usually require manual control or preprogramming of the process parameters. Significantly, depositions result in very little heat input because heat is generated during less than 1% of a weld cycle and dissipated during 99% of the cycle. ESD coatings are extremely dense and metallurgically bonded to the workpiece.
One of the distinguishing aspects of ESD, as compared to other arc-welding processes, is that the electrode contacts the surface rather than maintaining a stand-off distance to control the arc. Alternative deposition techniques for material repair and protection include high-velocity oxygen fuel (HVOF) thermal spray, physical vapor deposition (PVD), chemical vapor deposition (CVD), and electrolytic hard chrome (EHC) plating. In contrast to most of the above-mentioned techniques, which may produce mechanical or chemical bonds with a workpiece, ESD creates a true metallurgical bond while maintaining the workpiece at or near ambient temperatures.
One advantage of the ESD process is that the electrical pulse has a short duration, which produces nano-structured coatings with unique tribological and corrosion performance caused by the very rapid solidification of the deposited material. An additional benefit is that ESD does not call for special surface-preparation techniques, deposition chambers, spray booths, or particular operator protections for most materials. Perhaps most significantly, the process releases very little, if any, hazardous wastes, fumes, or effluents. The environmental compatibility of the ESD process is in sharp contrast to EHC plating, which the Department of Defense currently employs at virtually every repair depot.
EHC plating utilizes chromium in the hexavalent state (hex-Cr), which is a known carcinogen. Due to the hazards associated with hex-Cr, both the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA) strictly regulate air emission and permissible exposure limits. Furthermore, the EPA continues to propose lower allowable discharge concentrations, thereby significantly reducing the cost-effectiveness of EHC. Thus, significant motivation exists to implement alternative coating technologies that may lead to total replacement of chromium plating activities.
Because of its many advantages, ESD represents a viable, alternative deposition technique for material repair and protection. However, there are a number of critical variables that must be controlled for the process to result in acceptable coatings. Nearly all of these variables can be set or controlled by a skilled operator using prior experience and observing the spark characteristics during deposition. Therefore, the process has been used most frequently on external metal surfaces where the operator has clear visibility of, and easy access to, the workpiece. In general, ESD processes have been limited to applications where an operator can observe the weld arcs. When attempting to control the arc in applications involving non-line-of-sight coating of difficult-to-access geometries, a means and method to monitor and control the spark characteristics in a way that compensates for the operator""s lack of visibility must be developed and employed. Alternatively, the means and method could provide feedback that allows the operator to exercise the necessary process controls to maintain optimal spark characteristics. One of the primary and most troublesome variables that must be managed is the contact force between electrode tip and the workpiece. Too much or too little force renders the metallurgical structure of the final deposit unacceptable. Thus a need for an apparatus and a method for controlling ESD exists.
In view of the foregoing and other problems, disadvantages, and drawbacks of traditional coating technologies and ESD, the present invention has been devised. The invention resides in a novel apparatus for ESD and a method of controlling same. In one embodiment, the method for controlling ESD comprises the steps of providing an electrode tip, a conductive workpiece, a contact force between the electrode tip and the conductive workpiece, and a series of electrical energy pulses to the electrode tip. The method further comprises measuring a plurality of peak amplitudes from the series of electrical energy pulses, determining the maximum peak value out of the plurality of peak amplitudes, and obtaining an offset by comparing the maximum peak value to a target value, which correlates with an optimum contact force. Finally, the contact force is optimized according to the value of the offset.
The apparatus for controlled ESD comprises a consumable electrode tip electrically connected to an electrical energy wave generator and an electrical signal sensor, which connects electrically to a high-speed data acquisition card. A workpiece mounting system exists that allows the workpiece to contact the electrode tip, while an actuator provides relative motion between the mounting system and the electrode tip. A processor electrically connected to the high-speed data acquisition card and the actuator receives a feedback signal, compares the feedback signal to a set point, and transmits a drive signal to the actuator.
It is an object of the invention to provide the necessary feedback for automated adjustment of selected process parameters, such as contact force, required for achieving the desired metallurgy and application-specific surface characteristics.
It is another object to reproducibly coat non-line-of-sight and difficult-to-access surfaces using ESD. An example of one such surface is the inner surface of a gun barrel.
Yet another object of the present invention is to provide an environmentally-friendly coating alternative to traditional technologies such as EHC plating, PVD, and HVOF thermal spraying.
Still another object of the invention is to provide a portable ESD apparatus whereby an operator controls the contact force in response to a sensory stimulus emitted by the portable apparatus.
An additional object is to provide an efficient method, based on a design of experiments package, for determining the set points that will result in an optimum metallurgical structure of a deposit for a particular electrode tip/conductive workpiece combination.
Another additional object is to provide a method of using ESD in combination with three-dimensional models for forming a desired surface contour on a workpiece.